Recently, high power electronic circuits such as power modules, LEDs and the like have been used for various uses. An insulating substrate on which such a circuit is, for example, mountable is required. As such an insulating substrate, a ceramic substrate is generally used. Especially, a silicon nitride sintered substrate has a high mechanical strength. For example, Patent Document 1 discloses a silicon nitride sintered substrate having a low dielectric constant, a high air tightness and a high productivity.